This invention relates to a method and apparatus for submerged arc welding and, more particularly, to a welding process that uses four or five electrodes to efficiently manufacture large diameter steel pipes while maintaining high quality.
Large diameter steel pipes are typically formed into an O-shaped cross-section by various mechanical means. Once the pipes have been shaped it is desireable to seal the pipes' seam. One method that has been widely used to seal these seams is the submerged arc welding process. To improve the production efficiency in sealing these large diameter pipes, it is necessary to increase the speed of the submerged arc welding process because the speed relates directly to production efficiency. One method of increasing the efficiency of the welding process is using three electrodes as disclosed in U.S. Pat. Nos. 2,436,387, 3,336,460 and 3,456,089. However, the welding speeds obtained with these processes are not adequate to keep up with current demands.
As disclosed in U.S. Pat. No. 4,442,340 there are many factors which limit the welding speed, the most important of which are:
(1) insurance of sufficient penetration depth, and PA1 (2) avoidance of welding defects.
Welding penetration is a function of welding heat input and travel speed (electric current x voltage/welding speed). To achieve adequate penetration at high speeds the electric current must be increased. Current may be increased to a point after which magnetic interaction occurs resulting in arc instability. Arc instability leads to various discontinuities; of which the most pronounced at high travel speeds is "undercutting." Undercutting is a groove melted into the base metal adjacent to the toe of the weld which is left unfilled and is generally considered to be a welding defect.
One possible solution to increase welding speed without increasing individual electrode current is by adding additional electrodes into the process. However, multi-electrodes can cause electromagnetic interference of the electric current and the ground current. Accordingly, proper phase selection between electrodes and power source is required to control the interference. Thus, if current phasing selection is not correct, welding quality decreases and high speed multi-electrode welding cannot be accomplished.
Proper angling and spacing between electrodes must be maintained so that a condition known as "arc blow" does not occur. Arc blow reduces the efficiency of the process and decreases the quality of the weld. Arc blow occurs when there is strong magnetic deflection of the arcs away from the weld puddle which is caused by undesirable magnetic interaction between the electrodes.
A process that has attempted to increase the efficiency of welding pipes is a four electrodes submerged arc welding process disclosed in U.S. Pat. No. 4,442,340. This process increases the welding speed by applying alternating current to four electrodes. The four electrodes respond to the alternating current by applying an electromagnetic force within a predetermined range that is opposite to the welding direction. A drawback to this process is that to achieve adequate penetration, higher current must be applied to the lead electrode. This may lead to imbalances with the remaining electrodes. However, the welding speed obtained by this four electrode arc welding process still does not meet the welding speeds now required.